OBJECTIVE To examine the relationship between lower-extremity muscle cross-sectional area, muscle strength, specific torque, and age in ambulatory boys with Duchenne muscular dystrophy (DMD) compared with controls. METHODS Observational cross-sectional study. METHODS University research setting. METHODS Volunteer sample of boys with DMD (n=22) and healthy control boys (n=10), ages 5 through 14 years. METHODS Not applicable. METHODS Maximal muscle cross-sectional area (CSA(max)) assessed by magnetic resonance imaging of quadriceps, plantarflexors (PFs) and dorsiflexors (DFs), peak isometric torque from dynamometry, and timed functional tests. RESULTS The average CSA(max) of the triceps surae muscle group was approximately 60% higher in boys with DMD compared with controls (39.1+/-13.6 cm(2) vs 24.5+/-9.3 cm(2); P=.002), while the tibialis anterior muscle showed age-appropriate increases in CSA(max). The increase in quadriceps CSA(max) was also distinctly different in boys with DMD compared with controls. Specific torque (ie, peak torque/CSA(max)) was impaired in all 3 muscles groups, with the knee extensor (KE) and PF muscles showing 4-fold, and the DF muscles 2-fold, higher values in controls compared with boys with DMD. Large age-related gains in specific torque were observed in all 3 muscle groups of control subjects, which were absent in ambulatory boys with DMD. Correlations were observed between performance on functional tasks and quadriceps and PF torque production (r=-.45 to -.57, P<.05), but not with DF strength. CONCLUSIONS Age-related changes in muscle cross-sectional area and specific torque production in lower-extremity muscles showed distinctly different patterns in the KE, PF, and DF muscles of boys with DMD compared with controls.